Do you have an Android v. 8 phone? I have the feeling that they have a put a bug in one of the latest updates that corrupts something and stops the sync from working. I also got the same very unhelpful answer from support and in spite of me telling them how useless and upsetting their reply is it seems that they stick to it...
Well, after looking at different answers in the forum and doing the usual process you describe (and even trying syncing using a different device, which by the way made no difference, even though it took me a few hours of my life that I'll never get back)… I got it working again by replacing the Fitbit in the app with the same Fitbit.
You first need to go to your Bluetooth settings and "forget" (delete) the entry for your fitbit device. Then you go to the app and add a new device and select the option to add your type of Fitbit. It will ask you if you want to replace it and you need to say yes. And then follow the usual process of pairing.
Today is the 3rd time I have had to do this in the last 20 days. It works but every time you loose some settings that you need to reconfigure and what's worse, it seems to need 48 hours for the sleep analysis to start working again each time, even though you are replacing a device with EXACTLY the same device and it has all the data it needs to do the analysis. Another bug, me thinks...
